Heater problem
1997 Mercedes Benz S420 V8 Automatic 176000 miles

Hi, i have a 97 mercedes benz s420 and my problem is that when i turn on the heat to HI and the fan to full, on the driver side vent i get good air flow and heat, in the middle left one i also get good air flow and heat however on the middle right i get low air flow and hardly any heat and on the passenger vent i dont get any air flow or heat. Please help me!!! Thanks